1. Field of the Invention
The present invention relates to a method of manufacturing a color-filter-fitted board for a liquid crystal display, in which two layers of sheet-like electrodes with a color filter intercalated therebetween are formed on the inner surface of a board composing liquid crystal cells.
2. Description of the Prior Art
With a conventional simple matrix driving type liquid crystal color display, it is required to provide a sheet-like transparent electrode on a color filter for driving liquid crystal cells, in order to improve displaying performance. In addition, it is also needed to make a sheet resistance of the transparent electrode as small as possible, in order that a duty ratio for driving the liquid crystal cells is larger. Although it is effective to heat up said transparent electrode after a film-making process, to reduce the sheet resistance of said transparent electrode, a conventional color filter can resist a maximum temperature of about 270.degree. C. At a temperature such as above, a maximum limit sheet resistance obtainable by forming a transparent electrode as thick as possible on the color filter is only about 20 .OMEGA. to 30 .OMEGA.. Therefore, as disclosed in the Unexamined Japanese Patent Application Laid Open No. 26628/88, the inventors of the present invention proposed a method to decrease the sheet resistance by providing transparent electrodes in two-layer composition.
FIG. 5 depicts part of said color-filter-fitted board with transparent electrodes in two-layer composition. In FIG. 5, numerals la and lb represent transparent electrodes in two-layer composition, inside of which a color filter 2 is intercalated. In said color filter 2, a plurality of contact holes (through-holes) 3 is formed. Through said contact holes 3, a first-layer (lower) transparent electrode 1a and a second-layer (upper) transparent electrode 1b are connected electrically.
FIG. 6(a) to 69c) is a view for showing a process of manufacturing a color-filter-fitted board in the configuration described above, in which a section shape of the electrode pattern shown in FIG. 5 in a longitudinal direction is illustrated. First, as shown in FIG. 6(a), a film of a first-layer transparent electrode la is formed on the inner surface of a glass board 4 and heated up satisfactorily before forming a color filter in order to decrease a sheet resistance. A photoresist 5 is then coated at a predetermined location for forming said contact holes 3, on which a color filter 2 is formed by an electro-deposition method as shown in FIG. 6(b) . Thereafter, as shown in FIG. 6(c), said photoresist 5 is removed thereby forming contact holes 3 on which a film of a second-layer transparent electrode 1b is formed.
With the color-filter-fitted board manufactured as described above, the second-layer transparent electrode 1b is electrically connected to the first-layer transparent electrode 1a through the contact holes 3, FIG. 7. That is, the first- and second-layer transparent electrodes 1a and 1b are connected parallel to each other. Consequently, an apparent resistance of the electrodes is substantially the same as that of the lower transparent electrode 1a, even if the sheet resistance of the upper transparent electrode 1b of the color filter 2 is not decreased.
In the above, it is required to make the size of said contact holes 3 such that no adverse affect is given to an indication quality of a display panel. Therefore, a diameter normally taken is about 20 .mu.m to 30 .mu.m. And, a method often employed conventionally to form microscopic contact holes 3 such as above in a satisfactory position accuracy is that, before forming a color filter 2 by an electro-deposition method, a photoresist 5 in the same shape as that of the contact holes 3 is pattern-formed on the transparent electrode 1a while peeling said photoresist 5 after completion of electro-depositing the color filter 2, as described above.
On the other hand, in order to have satisfactory color reproducibility using an electro-deposited color filter 2 such as described above, the film thickness of the color filter 2 is required to be more than about 1.7 .mu.m. However, if the photoresist 5 is coated as thick as or more than the above for forming a microscopic pattern, development irregularities, etc. occur resulting sometimes in that the contact holes 3 are not formed in correct shapes. If the film thickness of the photoresist 5 is made to for instance 1.0 .mu.m to 1.2 .mu.m approximately to prevent the irregularities, etc., the thickness is smaller than the film thickness of the color filter 2 while forming the color filter 2 in such a manner that the filter covers up the pattern of the photoresist 5. Thus, the dimension of an opening of the contact holes 3 may disadvantageously become small in excess.
Normally, the shape of a contact hole 3 is a closed figure such as a circle, ellipse or a rectangular. Further, making a film of the transparent electrode 1b comprising ITO, etc. is performed often in a direction normal to the glass board 4. Therefore, if an area of the opening of a contact hole 3 is not satisfactory because of the reason described above, making a film on a side surface of the contact hole 3 cannot be performed satisfactorily resulting sometimes in faulty connection between the upper and lower transparent electrodes, though no such problems will occur provided the contact holes 3 are correctly formed.
In these circumstances, another method shown in FIG. 8 may also be considered, in which the shape of the contact holes 3 is made continuous to the side surface of the electrode pattern. At that time, however, if a connection error occurs in adjacent two contact holes 3 (for instance, between Portions .alpha. and .beta.), no driving voltage is applied any longer to the transparent electrode lb on the color filter 2 between said contact holes. Consequently, the portions are disabled to illuminate during displaying. Particularly with a dot matrix LCD panel used for full-color indication, the number of indication pixels in use is tens of thousands or hundreds of thousands or even more. Therefore, it is difficult to eliminate such pixels as disabled for illumination provided the shape of contact holes 3 is used as shown in FIG. 8. FIG. 9 depicts a section shape of the 9--9 line in FIG. 8.
In the above, disability of illumination owing to faulty connection between said contact holes occurs only with the contact holes in the shape of FIG. 8, but does not occur with those in the shape of FIG. 5. This is because, even when faulty connection occurs between contact holes 3 of the latter, a driving voltage is transmitted through adjacent normal contact holes 3.
Since conventional color-filter-fitted boards for liquid crystal displays are manufactured as described above, the dimension of an opening of the contact holes becomes small in excess where the contact holes are formed in closed shapes. At that time, making a film of an electrode on the side surface of a contact hole is not satisfactorily performed, resulting often in faulty connection. Also, if the contact holes are shaped in such a manner as continued toward the side surface of the electrode pattern, some pixels are sometimes disabled to illuminate because of faulty connection, causing a disadvantageous problem that no normal displaying can be activated.
The present invention has been accomplished from the viewpoint of the problems as described above, and is intended to provide such a method of manufacturing a color-filter-fitted board for a liquid crystal display, with which the occurrence of faulty connection in the contact holes can be prevented and, even if faulty connection in part of the contact holes occurs, a driving voltage can be transmitted and, therefore, there are no non-illumination pixels while maintaining normal displaying at any time.